Consider peeling back the sticker in the center of the fan and if you can see the bearing in there add a drop of oil. I'm sure many fans will work just fine.
Ideally I think you want a "blower" and not a "fan". Fans have blades like a propellor - typically 5 blades. Blowers have typically 30 blades and use centrifugal force and can withstand much more back pressure. I don't know if german suppliers use different words for the 2 kinds of devices.
Take a look at this one, both fan's are connected in parallel:
Indeed, the output on the PCB is 12v.
To my opinion you'll only need 1 50mm 12v fan aiming at the stepper drivers (at least I have on my UMO for almost 2 years).
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